Phase shifters, interrogators, methods of shifting a phase angle of a signal, and methods of operating an interrogator

ABSTRACT

The present invention includes phase shifters, interrogators, methods of shifting a phase angle of a signal, and methods of operating an interrogator. One aspect of the present invention provides a phase shifter including a first power divider configured to receive a signal and provide plural quadrature components of the signal; plural mixers coupled with the first power divider and configured to scale the quadrature components using a phase shift angle; and a second power divider coupled with the mixers and configured to combine the scaled quadrature components to shift the phase angle of the input signal by the phase shift angle.

TECHNICAL FIELD

The present invention relates to phase shifters, interrogators, methodsof shifting a phase angle of a signal, and methods of operating aninterrogator.

BACKGROUND OF THE INVENTION

Electronic identification devices, such as radio frequencyidentification devices (RFIDs), are known in the art. Such devices aretypically used for inventory tracking. As large numbers of objects aremoved in inventory, product manufacturing, and merchandising operations,there is a continuous challenge to accurately monitor the location andflow of objects. Additionally, there is a continuing goal to determinethe location of objects is in an inexpensive and streamlined manner. Oneway of tracking objects is with an electronic identification system.

One presently available electronic identification system utilizes amagnetic coupling system. In some cases, an identification device may beprovided with a unique identification code in order to distinguishbetween a number of different devices. Typically, the devices areentirely passive (have no power supply), which results in a small andportable package. However, such identification systems are only capableof operation over a relatively short range, limited by the size of amagnetic field used to supply power to the devices and to communicatewith the devices.

Another type of wireless electronic identification system is an activewireless electronic identification system. Attention is directed towardscommonly assigned U.S. patent application Ser. No. 08/705,043, filedAug. 29, 1996, now U.S. Pat. No. 6,130,602, which issued on Oct. 10,2000, and incorporated herein by reference, which describes such activesystems in detail. One such system is sold by Micron CommunicationsInc., 3176 S. Denver Way, Boise, Id. 83705 under the trademarkMicrostamp Engine (™).

These systems include integrated circuit devices which include an activetransponder and are intended to be affixed to an object to be monitored.The devices are capable of receiving and processing instructionstransmitted by an interrogator. A device receives the instruction, ifwithin range, then processes the instruction and transmits a response,if appropriate. The interrogation signal and the responsive signal aretypically radio-frequency (RF) signals produced by an RF transmitter,circuit.

Because active devices have their own power sources, they do not need tobe in close proximity to an interrogator or reader to receive power viamagnetic coupling. Therefore, active transponder devices tend to be moresuitable for applications requiring tracking of a tagged device that maynot be in close proximity to an interrogator. For example, activetransponder devices tend to be more suitable for inventory control ortracking.

The active transponder is capable of using backscatter communicationtechniques in responding to an interrogator. The interrogator outputs apolling signal followed by a continuous wave (CW) signal. The integratedcircuit devices are configured to modulate the continuous wave signal inbackscatter communication configurations. This modulation typicallyincludes selective reflection of the continuous wave signal. Thereflected continuous wave signal includes the reply message from theremote devices which is demodulated by the interrogator.

Certain drawbacks have been identified with the use of backscattercommunication techniques. For example, the transmission of thecontinuous wave signal using the interrogator can desensitize thereceiver of the interrogator during reception thereby of reply signalsfrom associated remote devices. In particular, some of the continuouswave signal tends to bleed through to the received reply messages. Suchresults in degradation of wireless communications.

Systems have been provided which improve wireless communications withoutthe drawbacks associated with conventional devices. Variable phaseshifters can be used in such systems. However, conventional variablephase shifters are typically very expensive and typically only operatewithin a certain specified range, (e.g., 0 to 180 degrees).

SUMMARY OF THE INVENTION

The present invention includes variable phase shifters, interrogators,methods of shifting a phase angle of a signal, and methods of operatingan interrogator.

It is desired to reduce power within a modulated return link continuouswave signal of a coherent backscatter communication system including aninterrogator and at least one remote communication device. Exemplaryremote communication devices include remote intelligent communicationdevices and radio frequency identification devices (RFID) of electronicidentification systems.

An exemplary interrogator comprises a coherent interrogator configuredto provide backscatter communications. More specifically, theinterrogator is configured to output a forward link communication and awireless continuous wave signal using a transmitter. The interrogator isalso configured to output a local continuous wave signal to a receiverof the interrogator following transmission of the forward linkcommunication. Provision of the local signal enables coherent operationof the interrogator. The interrogator is operable to receive return linkcommunications from at least one remote communication device responsiveto transmission of the forward link wireless communication.

The interrogator preferably includes a receiver operable to reduce theamplitude of a carrier signal of the return link communication. Forbackscatter communications, the remote communication device isconfigured to modulate the continuous wave signal providing a carriercomponent and side band components. The receiver of the interrogator ispreferably configured to reduce the amplitude of the carrier componentwhile maintaining the amplitudes of the side band components.

Variable phase shifters are disclosed to adjust the phase angle of thelocal continuous wave signal using a determined phase shift angle toreduce bleed through. The determined phase shift angle may be variedduring operation of the interrogator. According to one aspect of thepresent invention, a phase shifter includes a power divider configuredto provide plural quadrature components of an input signal, such as thelocal continuous wave signal. Plural mixers are provided to scale thequadrature components using the phase shift angle. A second powerdivider is provided to combine the scaled quadrature components to shiftthe phase angle of the input signal by the phase shift angle.

Methods of certain aspects of the present invention provide shifting ofa phase angle of an input signal according to a phase shift angle. Amethod of one aspect includes providing the input signal into pluralcomponents. Thereafter, the components are scaled using the phase shiftangle and combined to shift the phase angle of the input signal by thephase shift angle.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below withreference to the following accompanying drawings.

FIG. 1 is a block diagram of an exemplary communication system.

FIG. 2 is a front view of a wireless remote communication deviceaccording to one embodiment.

FIG. 3 is a front view of an employee badge according to anotherembodiment.

FIG. 4 is a functional block diagram of an exemplary transponderincluded in the remote communication device of FIG. 2.

FIG. 5 is a functional block diagram of an exemplary interrogator of thecommunication system.

FIG. 6 is a functional block diagram of an RF section of theinterrogator.

FIG. 7 is a functional block diagram of an adaptive canceler of the RFsection.

FIG. 8 is a schematic diagram of amplitude detectors and an amplitudeadjuster according to one adaptive canceler configuration.

FIG. 9 is a graphical illustration of a summed return link communicationoutputted from the adaptive canceler.

FIG. 10 is a schematic diagram illustrating one configuration of anamplitude detector and a phase adjuster of the adaptive canceler.

FIG. 11 is a graphical illustration of a received return linkcommunication.

FIG. 12 is a graphical illustration of a summed return linkcommunication.

FIG. 13 is a diagrammatic representation of a forward link communicationand a return link communication within the communication system.

FIG. 14 is a circuit schematic showing a variable phase shifter used inthe adaptive canceler, in one embodiment, and which also has other uses.

FIG. 15 is a graphical illustration of a relationship between I and Qcomponents in the variable phase shifter of FIG. 14.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This disclosure of the invention is submitted in furtherance of theconstitutional purposes of the U.S. Patent Laws “to promote the progressof science and useful arts” (Article 1, Section 8).

FIG. 1 illustrates a wireless communication system 10 embodying theinvention. Communication system 10 comprises an electronicidentification, system in, the embodiment described herein. Further, thedescribed communication system 10 is configured for backscattercommunications as described in detail below. Other communicationprotocols are utilized in other embodiments.

The depicted communication system 10 includes at least one electronicwireless remote communication device 12 and an interrogator 26. Radiofrequency communications can occur intermediate remote communicationdevices 12 and interrogator 26 for use in identification systems andproduct monitoring systems as exemplary applications.

Devices 12 include radio frequency identification devices (RFID) orremote intelligent communication (RIC) devices in the embodimentsdescribed herein. Exemplary devices 12 are disclosed in U.S. patentapplication Ser. No. 08/705,043, filed Aug. 29, 1996. Plural wirelessremote communication devices 12 typically communicate with interrogator26 although only one such device 12 is illustrated in FIG. 1.

In one embodiment, wireless remote communication device 12 comprises awireless identification device such as the MicroStamp (™) integratedcircuit available from Micron Communications, Inc., 3176 S. Denver Way,Boise, Id. 83705. Such a remote communication device 12 can be referredto as a tag or card as illustrated and described below.

Although multiple communication devices 12 can be employed incommunication system. 10, there is typically no communication betweenmultiple devices 12. Instead, the multiple communication devices 12communicate with interrogator 26. Multiple communication devices 12 canbe used in the same field of interrogator 26 (i.e., within thecommunications range of interrogator 26). Similarly, multipleinterrogators 26 can be in proximity to one or more of devices 12.

The above described system 10 is advantageous over prior art devicesthat utilize magnetic field effect systems because, with system 10, agreater range can be achieved, and more information can be obtained(instead of just identification information). As a result, such a system10 can be used, for example, to monitor large warehouse inventorieshaving many unique products needing individual discrimination todetermine the presence of particular items within a large lot of taggedproducts.

Remote communication device 12 is configured to interface withinterrogator 26 using a wireless medium in one embodiment. Morespecifically, communications intermediate communication device 12 andinterrogator 26 occur via an electromagnetic link, such as an RF link(e.g., at microwave frequencies) in the described embodiment.Interrogator 26 is configured to output forward link wirelesscommunications 27. Further, interrogator 26 is operable to receive replyor return link wireless communications 29 from devices 12 responsive tothe outputting of forward link communication 27. In accordance with theabove, forward link communications and return link communicationscomprise wireless signals, such as radio frequency signals, in thedescribed embodiment. Other forms of electromagnetic communication, suchas infrared, acoustic, etc. are possible.

Interrogator unit 26 includes a plurality of antennas X1, R1, as well astransmitting and receiving circuitry, similar to that implemented indevices 12. Antenna X1 comprises a transmit antenna and antenna R1comprises a receive antenna individually connected to interrogator 26.

In operation, interrogator 26 transmits the interrogation command orforward link communication signal 27 via antenna X1. Communicationdevice 12 is operable to receive the incoming forward link signal. Uponreceiving signal 27, communication device 12 is operable to respond bycommunicating the responsive reply or return link communication signal29. Communications of system 10 are described in greater detail below.

In one embodiment, responsive signal 29 is encoded with information thatuniquely identifies, or labels the particular device 12 that istransmitting, so as to identify any object, animal, or person with whichcommunication device 12 is associated.

More specifically, remote device 12 is configured to output anidentification signal within reply link communication 29 responsive toreceiving forward link wireless communication 27. Interrogator 26 isconfigured to receive and recognize the identification signal within thereturn or reply link communication 29. The identification signal can beutilized to identify the particular transmitting communication device12.

Referring to FIG. 2, one embodiment of remote communication device 12 isillustrated. The depicted communication device 12 includes a transponder16 having a receiver and a transmitter as described below. Communicationdevice 12 further includes a power source 18 connected to transponder 16to supply operational power to transponder 16. In the illustratedembodiment, transponder 16 is in the form of an integrated circuit 19.However, in alternative embodiments, all of the circuitry of transponder16 is not necessarily all included in integrated circuit 19.

Power source 18 is a thin film battery in the illustrated embodiment,however, in alternative embodiments, other forms of power sources can beemployed. If the power source 18 is a battery, the battery can take anysuitable form. Preferably, the battery type will be selected dependingon weight, size, and life requirements for a particular application. Inone embodiment, battery 18 is a thin profile button-type cell forming asmall, thin energy cell more commonly utilized in watches and smallelectronic devices requiring a thin profile. A conventional button-typecell has a pair of electrodes, an anode formed by one face and a cathodeformed by an opposite face. In an alternative embodiment, the batterycomprises a series connected pair of button type cells.

Communication device 12 further includes at least one antenna connectedto transponder 16 for wireless transmission and reception. In theillustrated embodiment, communication device 12 includes at least onereceive antenna 44 connected to transponder 16 for radio frequencyreception by transponder 16, and at least one transmit antenna 46connected to transponder 16 for radio frequency transmission bytransponder 16. The described receive antenna 44 comprises a loopantenna and the transmit antenna 46 comprises a dipole antenna.

Communication device 12 can be included in any appropriate housing orpackaging. FIG. 2 shows but one example of a housing in the form of aminiature housing 11 encasing device 12 to define a tag which can besupported by an object (e.g., hung from an object, affixed to an object,etc.).

Referring to FIG. 3, an alternative housing is illustrated. FIG. 3 showsa housing in the form of a card 13. Card 13 preferably comprises plasticor other suitable material. Plastic card 13 houses communication device12 to define an employee identification badge including thecommunication device 12. In one embodiment, the front face of card 13has visual identification features such as an employee photograph or afingerprint in addition to identifying text.

Although two particular types of housings have been disclosed, thecommunication device 12 can be included in any appropriate housing.Communication device 12 is preferably of a small size that lends itselfto applications employing small housings, such as cards, miniature tags,etc. Larger housings can also be employed. The communication device 12,provided in any appropriate housing, can be supported from or attachedto an object in any desired manner.

FIG. 4 is a high level circuit schematic of transponder 16 utilized inthe devices of FIGS. 1-3. In the embodiment shown in FIG. 4, transponder16 is implemented within monolithic integrated circuit 19. In theillustrated embodiment, integrated circuit 19 comprises a single die,having a size of 209×116 mils², including a receiver 30, transmitter 32,microcontroller or microprocessor 34, a wake up timer and logic circuit36, a clock recovery and data recovery circuit 38, and a bias voltageand current generator 42. Integrated circuit 19 preferably comprises asmall outline integrated circuit (SOIC) package. Receiver 30 andtransmitter 32 comprise wireless communication circuitry configured tocommunicate wireless signals.

In one embodiment, communication devices 12 switch between a “sleep”mode of operation, and higher power modes to conserve energy and extendbattery life during periods of time where no interrogation signal 27 isreceived by devices 12, using the wake up timer and logic circuitry 36.

In one embodiment, a spread spectrum processing circuit 40 is includedin transponder 16. In this embodiment, signals transmitted and receivedby interrogator 26 and signals transmitted and received by communicationdevice 12 are modulated spread spectrum signals. Many modulationtechniques minimize required transmission bandwidth. However, the spreadspectrum modulation techniques employed in the illustrated embodimentrequire a transmission bandwidth that is up to several orders ofmagnitude greater than the minimum required signal bandwidth. Althoughspread spectrum modulation techniques are bandwidth inefficient insingle user applications, they are advantageous where there are multipleusers, as is the case with the preferred radio frequency identificationcommunication system 10 of the present invention.

The spread spectrum modulation technique of the illustrated embodimentis advantageous because the interrogator signal can be distinguishedfrom other signals (e.g., radar, microwave ovens, etc.) operating at thesame frequency. The spread spectrum signals transmitted by communicationdevice 12 and interrogator 26 are pseudo random and have noise-likeproperties when compared with the digital command or reply. Theillustrated embodiment employs direct sequence spread spectrum (DSSS)modulation.

In operation, interrogator 26 sends out a command that is spread arounda certain center frequency (e.g, 2.44 GHz). After the interrogatortransmits the command, and is expecting a response, the interrogatorswitches to a continuous wave (CW) mode for backscatter communications.In the continuous wave mode, interrogator 26 does not transmit anyinformation. Instead, the interrogator just transmits a radio frequencycontinuous wave signal. In the described embodiment, the continuous wavesignal comprises a radio frequency 2.44 GHz carrier signal. In otherwords, the continuous wave signal transmitted by interrogator 26 is notmodulated. After communication device 12 receives the forward linkcommunication from interrogator 26, communication device 12 processesthe command.

If communication device 12 is operating in a backscatter mode, device 12modulates the continuous wave signal providing a modulated continuouswave signal to communicate return link communication 29 responsive toreception of forward communication signal 27. Communication device 12may modulate the continuous wave signal according to a subcarrier ormodulation signal. Modulation by device 12 comprises selectivereflection of the continuous wave signal. In particular, device 12alternately reflects or does not reflect the continuous wave signal fromthe interrogator to send its reply. For example, in the illustratedembodiment, two halves of a dipole antenna are either shorted togetheror isolated from each other to send a reply. Alternatively,communication device 12 can communicate in an active mode.

The modulated continuous wave signal communicated from device 12comprises a carrier component and plural side band components about thecarrier component resulting from the modulation. More specifically, themodulated continuous wave signal output from device 12 includes a radiofrequency continuous wave signal having a first frequency (2.44 GHz),also referred to as a carrier component, and a subcarrier modulationsignal having a different frequency (e.g., 600 kHz) and which providesthe side band components. In particular, the side band components are at+/−600 kHz of the carrier component. The carrier and side bandcomponents are illustrated in FIG. 11 and FIG. 12.

In one embodiment, the clock for transponder 16 is extracted from theincoming message itself by clock recovery and data recovery circuitry38. This clock is recovered from the incoming message, and used fortiming for microcontroller 34 and all the other clock circuitry on thechip, and also for deriving the transmitter carrier or the subcarrier,depending on whether the transmitter is operating in active mode orbackscatter mode.

In addition to recovering a clock, the clock recovery and data recoverycircuit 38 also performs data recovery on valid incoming signals. Thevalid spread spectrum incoming signal is passed through the spreadspectrum processing circuit 40, and the spread spectrum processingcircuit 40 extracts the actual ones and zeros of data from the incomingsignal. More particularly, the spread spectrum processing circuit 40takes chips from the spread spectrum signal, and reduces individualthirty-one chip sections down to a bit of one or zero, which is passedto microcontroller 34.

Microcontroller 34 includes a serial processor, or I/O facility thatreceives the bits from spread spectrum processing circuit 40. Themicrocontroller 34 performs further error correction. More particularly,a modified hamming code is employed, where each eight bits of data isaccompanied by five check bits used by the microcontroller 34 for errorcorrection. Microcontroller 34 further includes a memory, and afterperforming the data correction, microcontroller 34 stores bytes of thedata bits in memory. These bytes contain a command sent by theinterrogator 26. Microcontroller 34 is configured to respond to thecommand.

For example, interrogator 26 may send a command requesting that anycommunication device 12 in the field respond with the device'sidentification number. Status information can also be returned tointerrogator 26 from communication devices 12.

Communications from interrogator 26 (i.e., forward link communications)and devices 12 (i.e., return link communications) have a similar format.Exemplary communications are discussed below with reference to FIG. 13.More particularly, the forward and reply communications individuallyinclude a calibration period, preamble, and Barker or start code whichare followed by actual data in the described embodiment. The incomingforward link message and outgoing reply preferably also include a checksum or redundancy code so that transponder 16 or interrogator 26 canconfirm receipt of the entire message or reply.

Communication devices 12 typically include an identification sequenceidentifying the particular tag or device 12 sending the reply. Suchimplements the identification operations of communication system 10.

After sending a command, interrogator 26 sends the unmodulatedcontinuous wave signal. Return link data can be Differential Phase ShiftKey (DPSK) modulated onto the continuous wave signal using a square wavesubcarrier with a frequency of approximately 600 kHz (e.g., 596.1 kHz inone embodiment). A data 0 corresponds to one phase and data 1corresponds to another, shifted 180 degrees from the first phase.

The subcarrier or modulation signal is used to modulate antennaimpedance of transponder 16 and generate the modulated continuous wavesignal. For a simple dipole, a switch between the two halves of thedipole antenna is opened and closed. When the switch is closed, theantenna becomes the electrical equivalent of a single half-wavelengthantenna that reflects a portion of the power being transmitted by theinterrogator. When the switch is open, the antenna becomes theelectrical equivalent of two quarter-wavelength antennas that reflectvery little of the power transmitted by the interrogator. In oneembodiment, the dipole antenna is a printed microstrip half wavelengthdipole antenna.

Referring to FIG. 5, one embodiment of interrogator 26 is illustrated.The depicted interrogator 26 includes a microcontroller 70, a fieldprogrammable gate array (FPGA) 72, and RF section 74. In the depictedembodiment, microcontroller 70 comprises a MC68340 microcontrolleravailable from Motorola, Inc. FPGA 72 comprises an XC4028 deviceavailable from Xilinx, Inc. Further details of components 70, 72, and 74are described below.

RAM 76, EPROM 78 and flash memory 80 are coupled with microcontroller 70in the depicted embodiment. Microcontroller 70 is configured to accessan applications program for controlling the interrogator 26 andinterpreting responses from devices 12. The processor of microcontroller70 is configured to control communication operations with remotecommunication devices 12 during normal modes of operation. Theapplications program can also include a library of radio frequencyidentification device applications or functions. These functions effectradio frequency communications between interrogator 26 and communicationdevice 12.

RF section 74 is configured to handle wireless (e.g., radio frequency)communications with remote communication devices 12. DPSK modulationtechniques can be utilized for communications intermediate devices 12and interrogator 26. RF section 74 can include downconversion circuitryfor generating in-phase (I) and quadrature (Q) signals which contain theDPSK modulated subcarrier for application to FPGA 72 during return linkcommunications.

Plural antennas, including a transmit antenna X1 and a receive antennaR1 are coupled with RF section 74 for wireless RF communications. PluralRF transmit (TX) ports and RF receive (RX) ports (not shown) are coupledwith RF section 74 in a preferred embodiment. Provision of plural TXports and RX ports enables interrogator 26 to minimize the effects ofmultipath when communicating with plural remote communication devices12.

Analog to digital converters 82, 84 provide received analog RF signalsinto a digital format for application to FPGA 72. In particular, analogto digital converters 82, 84 are implemented intermediate FPGA 72 and RFsection 74 for both in-phase (I) and quadrature (Q) communication lines.An additional connection 85 is provided intermediate FPGA 72 and RFsection 74. Digital signals output from FPGA 72 via connection 85 areconverted to RF signals by RF section 74. Connection 85 can be utilizedto transmit phase lock loop (PLL) information, antenna diversityselection information and other necessary communication information.During forward link communications, FPGA 72 is configured to formatcommunication packets received from microcontroller 70 into a properformat for application to RF section 74 for communication.

FPGA 72 is configured to demodulate return link communications receivedfrom remote communication devices 12 via RF section 74. FPGA 72 isconfigured in the described embodiment to perform I and Q combinationoperations during receive operations. The described FPGA 74 furtherincludes delay and multiplication circuitry to remove the subcarrier.FPGA 74 can also include bit synchronization circuitry and lockdetection circuitry. Data, clock and lock detection signals generatedwithin FPGA 74 are applied to microcontroller 70 for processing in thedescribed embodiment.

Microcontroller 70 is configured to control operations of interrogator26 including outputting of forward link communications and receivingreply link communications. EPROM 78 is configured to store original codeand settings selected for the particular application of communicationsystem 10. Flash memory 80 is configured to receive software codeupdates which may be forwarded to interrogator 26.

RAM device 76 is configured to store data during operations ofcommunication system 10. Such data can include information regardingcommunications with associated remote communication devices 12 andstatus information of interrogator 26 during normal modes of operation.

Referring to FIG. 6, an exemplary embodiment of RF circuitry 74 isillustrated. The depicted RF circuitry 74 includes a transmit path 86and a receive path 87. In the depicted embodiment, RF section 74includes a transmitter 90, coupler 91 and power amplifier 92 withintransmit data path 86. Receive path 87 includes a receiver 95 comprisingprocessing circuitry 96 and an adaptive canceler 97 in the depictedembodiment.

Communication paths 86, 87 are coupled with respective antennas X1, R1.Transmit path 86 is additionally coupled with FPGA 72 via connection 85.Receive path 87 is coupled with analog-to-digital converters 82, 84 viathe I, and Q connection lines.

During communication operations, transmitter 90 is configured to outputa radio frequency wireless forward link communication 27 and a radiofrequency wireless continuous wave signal using coupler 91 and antennaX1. Further, transmitter 90 is also configured to output a localcontinuous wave signal using coupler 91. Transmitter 90 is preferablyconfigured to simultaneously output the wireless continuous wave signalusing antenna X1, and the local continuous wave signal using coupler 91.The wireless continuous wave signal transmitted via antenna X1 and thelocal continuous wave signal provided to receiver 95 via coupler 91 havea common frequency (e.g., 2.44 GHz in the described embodiment).

Receiver 95 is operable to receive the return link communications 29from at least one remote communication device 12 using antenna R1. Asdescribed in detail below, adaptive canceler 97 of receiver 95 isconfigured to receive the local continuous wave signal from coupler 91.Provision of the local signal provides a coherent backscatterinterrogator 26 including a coherent transmitter 90 and receiver 95.

As previously described, return link communication 29 comprises amodulated radio frequency continuous wave signal in the describedembodiment. The modulated signal comprises a carrier signal located atthe frequency of the wireless continuous wave signal (e.g., 2.44 GHz),and side bands located at +/−600 kHz about the frequency of the carriersignal. In the described embodiment, receiver 95 is configured to reducethe power or amplitudes of the return link communication. Morespecifically, receiver 95 is configured to reduce the power or amplitudeof the carrier signal of the return link communication.

In one embodiment, receiver 95 is operable to reduce the amplitude ofthe return link communication comprising the modulated continuous wavesignal using the local continuous wave signal. More specifically,receiver 95 is configured to reduce the amplitude of the return linkcommunications received by antenna R1 at the common frequency of thecontinuous wave signals in the described embodiment.

As described in detail below, receiver 95 is configured to receive thelocal continuous wave signal from coupler 91 and adjust the amplitudeand phase of the local continuous wave signal. Such adjustment providesan adjusted continuous wave signal. In particular, the amplitude of thelocal continuous wave signal is adjusted responsive to the amplitude ofthe modulated continuous wave signal. Preferably, the amplitude of thelocal continuous wave signal is adjusted to match the amplitude of thereceived return link communication. The amplitude of the localcontinuous wave signal is adjusted before adjustment of the phase of thelocal continuous wave signal in the described embodiment. Followingamplitude and phase adjustment, receiver 95 is configured to sum theadjusted continuous wave signal with the modulated continuous wavesignal. Thereafter, the summed return link communication having areduced amplitude at the frequency of the wireless continuous wavesignal is applied to processing circuitry 96.

Referring to FIG. 7, one embodiment of adaptive canceler 97 isillustrated. Adaptive canceler 97 is configured to reduce the amplitudeof return link communications 29. More specifically, during backscattercommunications, receive path 87 is susceptible to bleed through of thewireless continuous wave signal transmitted via antenna X1. Morespecifically, the wireless continuous wave signal communicated viatransmit antenna X1 can saturate the front end of receiver 95. Thisleakage can desensitize receiver 95 and reduce the quality of wirelesscommunications of interrogator 26 with remote communication devices 12.

Adaptive canceler 97 utilizes the local continuous wave signal receivedfrom transmitter 90 and coupler 91 to reduce the amplitude of the returnlink communication received by antenna R1 at the frequency of thewireless continuous wave signal transmitted via antenna X1.

As previously described, transmitter 90 is configured to output localand wireless continuous wave signals using coupler 91. Initially, thelocal continuous wave signal is applied to a variable attenuator 105within adaptive canceler 97. In the described embodiment, variableattenuator 105 comprises a voltage controlled attenuator. Variableattenuator 105 is configured to adjust the amplitude of the localcontinuous wave signal responsive to an external control signaldiscussed below.

Variable attenuator 105 outputs an amplitude adjusted local continuouswave signal. The amplitude adjusted local continuous wave signal isapplied to a phase shifter 106. Phase shifter 106 preferably comprises a360° phase shifter configured to provide an appropriate phase shift ofthe amplitude adjusted local continuous wave signal. Phase shifter 106outputs an amplitude and phase adjusted local continuous wave signalwhich is also referred to as the adjusted continuous wave signal. Phaseshifter 106 is controllable via an external control signal describedbelow.

The amplitude and phase adjusted local continuous wave signal outputfrom phase shifter 106 is supplied to a power divider 107. Power divider107 operates to apply the signal to a detector 108 and coupler 109.Detector 108 is operable to measure the amplitude of the adjusted localsignal and apply an output signal to an amplitude adjuster 110.

Return link communication 29 received via antenna R1 is applied to acoupler 114. Coupler 114 applies the received return link communication29 to coupler 109 and an amplitude detector 115. Detector 115 isconfigured to measure the amplitude of the received return linkcommunication 29.

Referring to FIG. 8, exemplary embodiments of amplitude adjuster 110 anddetectors 108, 115 are illustrated. Detectors 108, 115 individuallycomprise discrete components including diodes, resistors and capacitors.Detectors 108, 115 are configured to measure the amplitude of therespective adjusted continuous wave signal and the modulated continuouswave signal.

The measured amplitude values are applied to amplitude adjuster 110which comprises a feedback amplifier configuration in the depictedembodiment. The illustrated analog implementation of amplitude adjuster110 is configured to drive variable attenuator 105 to equalize theamplitudes of the adjusted continuous wave signal and the modulatedcontinuous wave signal. Amplitude adjuster 110 is configured to comparethe amplitudes of the adjusted continuous wave signal and the receivedreturn link communication comprising the modulated continuous wavesignal. Thereafter, amplitude adjuster 110 is operable to output acontrol signal to variable attenuator 105 to match the amplitudes of therespective signals. Other configurations of amplitude adjuster 110 arepossible.

Referring again to FIG. 7, coupler 109 is configured to sum the adjustedcontinuous wave signal and the received modulated continuous wave signalto reduce the amplitude of the modulated continuous wave signal. Thesummed continuous wave signal or return link communication is applied toa coupler 118. Coupler 118 is configured to apply the summed signal tolow noise amplifier (LNA) 119 and amplitude detector 120. Amplitudedetector 120 is configured to measure the amplitude of the summed signaland apply an output signal to a phase adjuster 121.

Phase adjuster 121 is controllable responsive to amplitude adjuster 110.Once amplitude adjuster 110 and variable attenuator 105 have matched theamplitudes of the adjusted continuous wave signal and the receivedreturn link communication, amplitude adjuster indicates the match tophase adjuster 121 via a connection 122. Thereafter, phase adjuster 121operates to select an appropriate phase shift of the amplitude adjustedlocal continuous wave signal.

In the described embodiment, phase adjuster 121 is configured to searchacross 360° of possible phase adjustments to detect a phase adjustmentof the local continuous wave signal which provides maximum reduction ofamplitude of the received modulated continuous wave signal at thecontinuous wave signal frequency. In particular, adaptive canceler 97adjusts the phase of the local continuous wave signal following matchingof amplitudes of the local continuous wave signal and the receivedmodulated continuous wave signal as indicated via connection 122.

Referring to FIG. 9, a graphical illustration of the amplitude of thesummed return link communication, represented by reference numeral 136,is illustrated with respect to corresponding plural phase adjustments ofthe local continuous wave signal. In the depicted illustration, it isshown that a local minimum value 130 corresponds to approximately 150°.For such a situation following searching of 360°, phase adjuster 121will apply an appropriate control signal to phase shifter 106 toimplement the desired phase shift of approximately 150° to minimize theamplitude of the bleed through of the wireless continuous wave signalwithin the received return link communication.

Referring again to FIG. 7, the summed return link communication isapplied to low noise amplifier 119 and processing circuitry 96. Phaseadjuster 121 is operable to continuously monitor the amplitude of thesummed return link communication and provide appropriate adjustmentsusing control signals applied to phase shifter 106 to minimize theamplitude of the continuous wave signal within the summed return linkcommunication applied to LNA 119.

Referring to FIG. 10, exemplary embodiments of amplitude detector 120and phase adjuster 121 are illustrated. Amplitude detector 120 includesdiscrete components comprising a diode, resistor and capacitor.

Phase adjuster 121 comprises an analog-to-digital converter 124,processor 125 and digital-to-analog converter 126. Processor 125 can beconfigured to execute appropriate algorithms to implement sequentialphase shifts of the local signal from 0° to 360°. The incremental stepsizes can be adjusted. Therefore, processor 125 can compare theamplitudes of the summed return link communication signal responsive tovarious phase adjustments implemented by phase shifter 106. Followingselection of an appropriate phase shift, phase adjuster 121 can continueto monitor the amplitude of the summed return link communication andupdate the phase shift as necessary to maintain maximum reduction of thecontinuous wave signal within the return link communication duringcommunications. The depicted configurations of detector 120 and phaseadjuster 121 are illustrative and other configurations can be utilized.

Referring to FIG. 11 and FIG. 12, the received return link communicationapplied to adaptive canceler 97 and the summed return link communicationoutput from adaptive canceler 97 are illustrated. The received returnlink communication comprising the modulated continuous wave signal isillustrated as signal 132 in FIG. 11. The summed return linkcommunication is represented by signal 136 of FIG. 12.

Signal 132 comprises a carrier component 133 and side band components134. In the described embodiment, carrier 133 is centered at a frequencyof 2.44 GHz and subcarrier side band components 134 are depicted atlocations +/−600 kHz of the carrier component 133. Signal 136 similarlycomprises a carrier component 137 and side band components 138. Signal136 includes carrier component 137 at a frequency of 2.44 GHz and sideband components 138 at locations +/−600 kHz of the carrier component137.

As illustrated, the output summed return link communication signal 136has a carrier component 137 having a reduced amplitude compared with thecarrier component 133 of the received return link communication signal132. Preferably, the amplitude of side band components 138 of summedreturn link communication signal 136 are maintained during the reductionof amplitude of the carrier component 137 as illustrated in FIG. 11 andFIG. 12.

In the depicted illustrations of FIG. 11 and FIG. 12, carrier component137 of signal 136 is approximately 20 dBm less than carrier component133 of received return link communication 132. Such indicates thereduction of amplitude of the return link communication signal at thefrequency of the wireless continuous wave signal (e.g., 2.44 GHz)utilizing adaptive canceler 97.

Referring to FIG. 13, a diagrammatic illustration of forward linkcommunication 27 and return link communication 29 is shown. Initially,forward link communication 27 is communicated using transmit antenna X1of interrogator 26. Following an intermediate delay or guard band,return link communication 29 corresponding to remote communicationdevice 12 is communicated.

Individual return link communications 29 include a calibration period140 followed by a preamble 141 and actual data 142. Matching ofamplitudes of the local continuous wave signal and the received returnlink communication and cycling through phases from 0 to 360° utilizingphase adjuster 121 and phase shifter 106 preferably occurs duringcalibration period 140. The minimum level 130 within the summed returnlink communication signal is preferably determined during calibrationperiod 140.

Preamble 141 can be utilized to synchronize the processing circuitry 96of receiver 95 with the actual return link communication 29 beingreceived. Thereafter, data 142 communicated from remote communicationdevice 12 is received. Adaptive canceler 97 is configured to makeadjustments as necessary to the amplitude and phase of the localcontinuous signal during preamble period 141 and data period 142 tomaintain maximum reduction of the continuous wave signal within thereceived return link communication 29.

FIG. 14 illustrates a variable phase shifter 106 in accordance with oneembodiment of the invention. Conventional phase shifters available inthe marketplace could be employed; however, these are extremelyexpensive. The phase shifter 106 illustrated in FIG. 14 is aninexpensive alternative. Additionally, the phase shifter 106 providesphase shifts of anywhere from 0 to 360 degrees.

The depicted phase shifter 106 uses a commonly available part known asan IQ upconverter or IQ downconverter 201. The IQ upconverter or IQdownconverter 201 includes a first power divider 200 defining an input210 and having two outputs 212 and 214, a second power divider 208defining an output 202 and having two inputs 204, 206, and two mixers216 and 218.

The mixer 216 is coupled between the output 212 of the first powerdivider 208 and the input 204 of the second power divider 200. The mixer218 is coupled between the output 214 of the first power divider 208 andthe input 206 of the second power divider 200.

In the described arrangement, first power divider 200 comprises a ninetydegree power divider and second power divider 208 comprises a zerodegree power divider. Power divider 200 receives an input signal havinga phase angle from input 210 which is the amplitude adjusted localcontinuous wave signal received from variable attenuator 105 previouslydescribed.

Power divider 200 provides a ninety degree phase shift to the inputsignal to provide a first component and a second component in accordancewith one embodiment of the invention. Inasmuch as power divider 200provides a ninety degree phase shift, first and second components of thereceived signal may be referred to as quadrature components. Inparticular, the first component from output 212 is indicated as a cosinecomponent (Cos(ωt)) and the second component from output 214 isindicated as a sine component (Sin(ωt)). The signal components Cos(ωt)and Sin(ωt) have a sine/cosine relationship as they are shifted ninetydegrees from each other. The quadrature signal components shifted ninetydegrees apart are applied to respective mixers 216, 218.

The depicted phase shifter 106 further includes plural digital-to-analog(D/A) converters 224, 226, I, Q drivers 220, 222, and storage device228. Phase adjuster 121 is coupled with storage device 228. A secondinput 203 of phase shifter 106 is provided intermediate phase adjuster121 and storage device 228.

As previously described, phase adjuster 121 is configured to calculate adesired phase shift angle (also referred to herein as Φ) and apply thephase shift angle to storage device 228. More specifically, phaseshifter 106 is configured to adjust the phase of the amplitude adjustedlocal continuous wave signal received from variable attenuator 105responsive to control signals from phase adjuster 121 and correspondingto the phase shift angle. Appropriate control signals are generatedwithin phase adjuster 121 to indicate the desired phase shift angle forshifting of the phase of the amplitude adjusted local continuous wavesignal. In particular, the control signals correspond to the desiredphase shift adjustment to provide the local minimum value 130 within thesummed return link communication signal as previously described. Thecontrol signals identifying the proper phase adjustment are applied tophase shifter 106.

Storage device 228 comprises a look-up table in an exemplary embodiment.Such a look-up table may be implemented within an EPROM in oneembodiment. Storage device 228 can have one degree resolution, or otherresolutions if desired. Storage device 228 is configured to store aplurality of sine values and cosine values, also referred to as I and Qdigital values. Further, storage device 228 is configured to output oneof the stored cosine values and one of the stored sine values to therespective D/A converters 224, 226 responsive to and corresponding tothe received phase shift angle determined by phase adjuster 121. Forexample, if a 45 degree phase shift is desired as indicated from phaseadjuster 121, storage device 228 outputs digital look-up table values of0.707, 0.707 (i.e., cosine and sine of 45 degrees) which are provided toD/A converters 224, 226.

Storage device 228 is coupled with D/A converters 224, 226 which in turnare coupled with respective I and Q drivers 220, 222. The cosine andsine digital values outputted from storage device 228 are converted toanalog voltages within D/A converters 224, 226. The corresponding analogvoltage signals from D/A converters 224, 226 are applied to respective Iand Q drivers 220, 222 to implement the proper phase shift within theoutputted signal to minimize bleed through.

I driver 220 is coupled to mixer 216 and Q driver 222 is coupled tomixer 218 as illustrated. Mixers 216, 218 are configured to scale therespective cosine and sine components of the input signal cos(ωt),sin(ωt) using the phase shift angle of phase adjuster 121. In thedescribed configuration, mixers 216, 218 individually act as multipliersand multiply the cosine and sine components of the input signal by therespective cosine and sine values from storage device 228 as provided toI and Q drivers 220, 222. More specifically, mixers 216, 218 multiplythe cosine and sine components cos(ωt), sin(ωt) of the input signal byvoltages outputted by the respective I driver 220 and the Q driver 222and corresponding to the cosine value and sine value outputted fromstorage device 228.

By adjusting the values outputted by the I driver 220 and the Q driver222, a phase shift of anywhere between 0 degrees and 360 degrees can beobtained. Exemplary phase adjustments are described hereafter. Becausethe input signal components cos(ωt) and sin(ωt) are ninety degrees outof phase, if combined at the second power divider 200 comprising a zerodegree power divider without use of multipliers 216, 218, the summedcomponents represented as vectors would have the same value as the inputsignal plus a ninety degree phase shift of the input signal.

Assume, for example, that the input signal has a constant phase and anamplitude of 1. If the output signal is desired to have the exact samephase, then the I driver 220 is set to provide one volt to the mixer 216and the Q driver 222 is set to provide zero volts to the mixer 218.Thus, the signal applied to output 202 would be the same as the signalreceived from input 210.

If a ninety degree phase shift is desired, the I driver 220 would be setto provide zero volts to mixer 216 so there is no signal at output 204,and the Q driver would be set to provide one volt to mixer 218 toproduce a ninety degree phase shifted value at the output 202. Toproduce a 45 degree phase shift, the I driver 220 is set to provide0.707 volts and the Q driver is set to provide 0.707 volts so by vectoraddition, the signal at the output 202 is shifted 45 degrees from thesignal at the input 210.

FIG. 15 illustrates the relationship between the I and Q signals appliedto the mixers 216, 218, respectively. The relationship is a sine/cosinerelationship. Appropriate I and Q values may be determined for any otherdesired degree phase shift (i.e., 0-360 degrees). For example, if a 45degree phase shift is desired (i.e., Φ equals 45 degrees), I is at 0.707(i.e., cosine Φ) while Q is at 0.707 (i.e., sine Φ) as determined withinstorage device 228. Such cosine and sine values are provided to therespective I and Q drivers 220, 222.

Referring again to FIG. 14, the signals outputted from mixers 216, 218may be referred to as scaled quadrature cosine and sine components,respectively. The scaled quadrature component from mixer 216 may beindicated as (Cos(Φ)Cos(ωt)) and the scaled quadrature component frommixer 218 may be indicated as (Sin(Φ)Sin(ωt)).

The scaled quadrature components are applied to second power divider208. Power divider 208 is configured to combine the first scaledquadrature component received from mixer 216 with the second scaledquadrature component received from mixer 218 to shift the phase angle ofthe local continuous wave signal by the phase shift angle received fromphase adjuster 121.

In general, the phase of the signal passing within phase shifter 106 maybe represented as I+Q where I=Cos(Φ)Cos(ωt) and Q=Sin(Φ)Sin(ωt). Powerdivider 208 is configured to add the scaled first and second quadraturecomponents received from mixers 216, 218 to implement phase shiftingoperations providing the adjusted continuous wave signal at output 202.The adjusted continuous wave signal having a phase angle shifted by thedesired phase shift angle Φ is outputted from phase shifter 106 and maybe applied via output 202 to power divider 107 and coupler 109.

As previously described, coupler 109 is configured to sum the adjustedcontinuous wave signal and the received modulated continuous wavesignal. Such reduces-the amplitude of the modulated continuous wavesignal at the frequency of the continuous wave to reduce bleed throughof the carrier signal.

In compliance with the statute, the invention has been described inlanguage more or less specific as to structural and methodical features.It is to be understood, however, that the invention is not limited tothe specific features shown and described, since the means hereindisclosed comprise preferred forms of putting the invention into effect.The invention is, therefore, claimed in any of its forms ormodifications within the proper scope of the appended claimsappropriately interpreted in accordance with the doctrine ofequivalents.

What is claimed is:
 1. An interrogator of a backscatter communicationsystem comprising: a transmitter configured to output a local continuouswave signal and a radio frequency continuous wave signal; and a receiverconfigured to receive the local continuous wave signal and a modulatedradio frequency continuous wave signal, the receiver including: a phaseshifter configured to adjust a phase angle of the local continuous wavesignal by a phase shift angle, the phase shifter including a first powerdivider configured to provide a first component and a second componentof the local continuous wave signal, plural mixers configured to scalethe first component and the second component using the phase shiftangle, and a second power divider configured to combine the scaled firstcomponent and the scaled second component to provide an adjustedcontinuous wave signal; and a coupler configured to combine the adjustedcontinuous wave signal and the modulated radio frequency continuous wavesignal.
 2. The interrogator according to claim 1 wherein the first powerdivider is configured to provide the signal into quadrature components.3. The interrogator according to claim 1 wherein the first power dividercomprises a ninety degree power divider configured to provide the signalinto a sine component and a cosine component.
 4. The interrogatoraccording to claim 1 further comprising a storage device configured tostore plural sine values and plural cosine values and output a sinevalue and a cosine value individually corresponding to the phase shiftangle.
 5. The interrogator according to claim 1 further comprising astorage device configured to store a sine value and a cosine valueindividually corresponding to the phase shift angle.
 6. The interrogatoraccording to claim 5 wherein the mixers are coupled with the storagedevice and individually configured to multiply one of the first andsecond components by one of the sine value and the cosine value.
 7. Theinterrogator according to claim 1 wherein the second power dividercomprises a zero degree power divider configured to add the scaled firstcomponent and the scaled second component.
 8. The system according toclaim 1 wherein the phase adjuster is configured to search a pluralityof different phase shift angles to select the phase shift angle.
 9. Amethod of operating a coherent interrogator of a backscattercommunication system comprising: outputting a radio frequency continuouswave signal; providing a local continuous wave signal; receiving amodulated continuous wave signal; providing a phase shift angle;adjusting the phase of the local continuous wave signal using the phaseshift angle to provide an adjusted continuous wave signal, the adjustingincluding: providing the local continuous wave signal into a firstcomponent and a second component; scaling the first component using thephase shift angle; scaling the second component using the phase shiftangle; and combining the first component and the second component afterthe scalings to shift the phase angle of the local continuous wavesignal by the phase shift angle; and combining the adjusted continuouswave signal and the modulated continuous wave signal.
 10. The methodaccording to claim 9 wherein the providing the signal into a firstcomponent and a second component comprises providing the signal intoquadrature components.
 11. The method according to claim 9 wherein theproviding the signal into a first component and a second componentcomprises providing the signal into a sine component and a cosinecomponent.
 12. The method according to claim 9 further comprisingstoring a plurality of sine values and cosine values and outputting asine value and a cosine value individually corresponding to the phaseshift angle.
 13. The method according to claim 9 further comprisingstoring a sine value and a cosine value individually corresponding tothe phase shift angle.
 14. The method according to claim 13 wherein thescalings individually comprise multiplying one of the first componentand the a second component by one of the sine value and the cosinevalue.
 15. The method according to claim 9 wherein the combiningcomprises adding the scaled first component and the scaled secondcomponent.
 16. The method according to claim 9 wherein the selectingcomprises selecting a plurality of different phase shift angles at aplurality of moments in time.